Unfortunately Slack does not support embedded links in it's response to apps. So when you schedule a message, we have no way to know if you've included a link in your message.
If you need to schedule a Slack message with an link included, the best option is to included the full URL of the link instead of embedding it in your text. Ie:
/schedule here is the website - www.google.com in 5 hours
Don't embed a URL using this option if you would like your Url to be included in your scheduled message: